Battery chargers and methods for charging of rechargeable batteries such as lithium-ion-type batteries are well known in the art.
A typical charging cycle of a lithium-ion battery comprises an initial phase in which the battery is charged with a constant charging current (CC). When the battery voltage reaches a predetermined threshold value a constant voltage phase (CV) begins. During the constant voltage phase, a charge controller maintains the battery voltage at a constant value, allowing the charging current to diminish as the battery continues to charge. The charging current is monitored, and the charging cycle is terminated when the charging current drops below a predetermined cut off value. Lithium ion batteries do not tolerate overcharging.
During the constant current phase, and in the beginning of the constant voltage phase, the charging level of the battery increases rapidly, whereas it increases slowly during the greater part of the constant voltage phase. An easy way of reducing the charging time is thus to increase the predetermined charging current cut off value. This leads to a non-fully charged battery however and thus a lower capacity.
When selecting charging characteristics, there has to be a balance between charging time requirements and battery capacity requirements.
In some applications, there is a considerable voltage drop between the charge controller and the battery. This is usually the case for robotic garden tools, since a resistive low voltage cable is generally arranged between the power source/charge controller and the charging station of the mower. If the voltage measured at the charge controller is considered to correspond to the battery voltage, the battery voltage will therefore be overestimated, and the constant current phase will be terminated unnecessarily early if this measured voltage is used as a cut-off trigger for the constant current phase. This means that a longer charging time or a lower charging level has to be accepted.
One way of compensating for this voltage drop is to have a separate power supply in the mower. However this would add cost and generate heat in the mower. Moreover it would add weight to the mower and consume space in it.
Another solution is to add separate sense cables between the battery and the charge controller. Such a solution would also add cost since more expensive cables and other additional components would have to be used.
In addition to finding an optimal balance between the battery capacity and the charging time, another aspect has to be taken into account for batteries in which a plurality of cells are connected in series. There are always variations in the parameters of the cells. This 15 will lead to imbalance, causing the capacity of the battery to drop. To eliminate the capacity drop, a cell balancing procedure needs to be performed from time to time. The cell balancing has to be performed when the charging current is low, such as when the battery is nearly fully charged. Thus, the battery needs to be fully charged now and then.
Thus, there is a need for a charging system and a charging method which enables fast charging of a lithium-ion battery, at a low cost, even if there is a considerable voltage drop between the power supply and the battery. The system and method shall also allow full charging with cell balancing at low cost.